It's hard to recommend one thing because there are so many options and they all have different trade-offs in terms of initial cost, ease-of-use, reliability, performance, etc.

A year or two back, I was able to get a brand-new fanless Intel N150 with 4x2.5G ports with 16 GB memory for about $150 from AliExpress. I run Proxmox on it, with OpnSense and a couple other things in virtual machines. These days, due to tariffs and the memory shortage, that is more like $440 now, unfortunately. I am kicking myself for not buying two, not so much because of the price increase, but because it would have come in handy multiple times to have a second one on-hand for random experiments.

Given that CPU performance does _not_ tend to be critical for firewall/NAS use cases, if I had to replace it tomorrow, I would go onto eBay and get the highest-spec'd used Dell or HP mini workstation I could find for $120 and plug in a USB3 1gig ethernet dongle for the WAN side.